The present invention is directed to an apparatus for controlling a current supply device which provides a supply current to a load. More specifically, the present invention is directed to an apparatus for controlling a current supply device which provides a supply current to a subscriber line in a telephone system.
It is desirable that a customer (such as a telephone operating company, a supplier of central office switching equipment or a supplier of private branch office exchange telephone equipment) be able to control and establish particular parameters in equipment supplying feed current to a loop such as a subscriber line associated with a telephone system. In particular, such parameters as feed resistance and limiting current, and the parameters establishing an anti-saturation region in the response of the system to varying loads on the subscriber line must be imposed appropriately to enable transmission of signals without distortion.
Prior art technology can effect such parameter setting, but a change of parameters requires a new mask for chip manufacture for each adjustment of a parameter. Thus, any parameter change must occur during chip manufacture, and no selection, or programmability capability is available to a user.
To provide such a programmable selection capability for the required parameters using prior art technology necessitates the employment of on-chip resistors to adjust the direct current (DC) parameters normally associated with subscriber line circuits. Such on-chip resistors in, for example, CMOS technology occupy a significant amount of space on the chip substrate, commonly called "real estate". The high values of resistance required to set the appropriate parameters for a subscriber loop, and their high real estate occupancy, militate against the provision of a bank of resistors which can be selected by software control to provide the desired programmability for customer customization of a chip to a particular telephone installation.
There is a need for a control circuit for controlling a current supply device for providing a supply current to a load in a telephone subscriber line which is compact and enables a user to employ software programming to select appropriate parameters without requiring a new mask for each change in parameter value. It is especially desirable that such a control circuit be implementable using CMOS technology.